Battery containers for batteries such as a primary battery like alkali-manganese battery, a secondary battery like nickel-cadmium battery, and a nickel-hydrogen battery, which is expected to be increasingly demanded as a new secondary battery, should be filled with a strong alkaline solution. They have been conventionally manufactured by a so-called after-plating method in which a cold rolled steel sheet is press-formed and subjected to a barrel plating or by a so-called prior-plating method in which a nickel plated steel sheet is pressed and formed into a battery container. Many proposals for improvement in these methods have been made, among which the present inventors have proposed an excellent surface-treated steel sheet used for a battery container with a low internal resistance. (International Publication No. WO95/11527)
Further, in recent years, as the press-forming method for manufacturing a battery container, DI (Drawing and Ironing) forming method has taken the place of the multi-step deep drawing method since DI forming method enables the container to be much more thinned so as to achieve an increased capacity of the battery (Publication of Patent applications No. HEI-7-99686). These DI forming method and DTR (Drawing Thin and Redrawing) forming method are advantageous, because they enable a battery container to be so formed so that the thickness of the side wall is thinner than that of the bottom portion. Accordingly, much more anode and cathode electrode active materials can be packed into the container so as to attain increased battery capacity and also attain enhanced pressure-proof strength of the battery because of a rather large thickness of the bottom portion of the container.
The DI forming method and DTR forming method are effective in achieving the increase in the battery capacity as mentioned above. However, as for the formability of a container, they have a disadvantage in the continuous formability because a material subjected to these forming methods has a greater deformation resistance compared with that used in the conventional multi-step deep drawing method.
More specifically, in a case where the material is inferior in the anti-powdering properties (the properties of a plating layer against being peeled off in a powdery state) when subjected to the cupping operation in the DI forming method or DTR forming method, the peeled-off powder will adhere to a die and punch during the ironing operation, with the result that the container suffers flaws on the side wall thereof. This happens in the deep drawing forming in the same way. Though, in the cases of the DI forming method and DTR forming method, the obtained container has a side wall having a small surface roughness and glossier appearance, so the above mentioned flaws appear rather conspicuous. Thus, the anti-powdering properties of a material is more significant in the cases of the DI forming method and DTR forming method. In addition, in the cases of the DI forming method and DTR forming method, the material and tools come into contact with each other at a higher contact pressure compared with that in the case of deep drawing method. Therefore, the material is required to have excellent smoothness in view of the lifetime of the tools. Thus, a material excellent in the anti-powdering properties is required to be used as a battery container.
However, the conventionally used material sheet has the surface to be the outer surface of a battery container provided with a bright nickel plating. This bright nickel plating has a problem that it shows poor anti-powdering properties when subjected to press-forming. Another problem is that the bright nickel plating includes sulfur-containing organic additives (sulfonic acid comprising a=C—SO2-group, for example) for micronizing electrodeposited crystal particles and this sulfur is adsorbed in the plating layer during the electrodeposition. When the material sheet is subjected to ironing and stretching in the DI forming process or DTR forming process, the temperature of the material is raised, which increases the brittleness of the plating layer due to sulfur. As a result, the anti-powdering properties of the material is further deteriorated.
With a view to solving the above mentioned problem, it is an object of the present invention to provide a battery container, of which the inner surface has a surface-treated layer having low internal resistance and of which the outer surface has a surface-treated layer of high quality and excellent in continuous formability.
It is also a technological object to provide a surface-treated steel sheet suitably used for forming the battery container.
It is a further technological object to improve the ability of a formed container to be pulled off from a punch (strippability) after DI forming or DTR forming. This is considered since the degree of difficulty in removing a formed container from the punch (strippability) in the final operation of the press-forming process is important in addition to the above mentioned anti-powdering properties. When being pulled off from the punch, the formed container has its end portion hooked to be removed from the punch. And when the container has poor strippability, it will frequently suffer breaks or tears at the end portion thereof, which may lower the productivity of the container.